(thûr'mə-klīn') A distinct layer in a large body of water, such as an ocean or lake, in which temperature changes more rapidly with depth than it does in the layers above or below. Thermoclines may be a permanent feature of the body of water in which they occur, or they may form temporarily in response to phenomena such as the solar heating of surface water during the day. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude and longitude, and local environmental conditions.

oceanic water layer in which water temperature decreases rapidly with increasing depth. A widespread permanent thermocline exists beneath the relatively warm, well-mixed surface layer, from depths of about 200 m (660 feet) to about 1,000 m (3,000 feet), in which interval temperatures diminish steadily. The deep waters below the thermocline layer decrease in temperature much more gradually toward the seafloor. In latitudes marked by distinct seasons, a seasonal thermocline at much shallower depths forms during the summer as a result of solar heating, and it is destroyed by diminished insolation and increased surface turbulence during the winter. Water density is governed by temperature and salinity; consequently, the thermocline coincides generally with the pycnocline, or layer in which density increases rapidly with depth. The middle layer of water in a lake or reservoir during the summer is also called a thermocline.